inabox/InABox.Avalonia/Components/ImageEditor/PanAndZoomGestureRecognizer.cs

using Avalonia;
using Avalonia.Input;
using Avalonia.Input.GestureRecognizers;
using Avalonia.Interactivity;

namespace InABox.Avalonia.Components.ImageEditing;

public class PanAndZoomEventArgs : RoutedEventArgs
{
    public PanAndZoomEventArgs(double scale, Point scaleOrigin, Vector pan) : base(PanAndZoomGestureRecognizer.PanAndZoomEvent)
    {
        Scale = scale;
        ScaleOrigin = scaleOrigin;
        Pan = pan;
    }

    public double Scale { get; } = 1;

    public Point ScaleOrigin { get; }

    public Vector Pan { get; set; }
}

public class PanAndZoomEndedEventArgs : RoutedEventArgs
{
    public PanAndZoomEndedEventArgs() : base(PanAndZoomGestureRecognizer.PanAndZoomEndedEvent)
    {
    }
}

// Adapted from the PinchGestureRecognizer built in to Avalonia, but with panning added.
public class PanAndZoomGestureRecognizer : GestureRecognizer
{
    public static readonly RoutedEvent<PanAndZoomEventArgs> PanAndZoomEvent =
        RoutedEvent.Register<PanAndZoomEventArgs>(
            "PanAndZoom", RoutingStrategies.Bubble, typeof(PanAndZoomGestureRecognizer));

    public static readonly RoutedEvent<PanAndZoomEndedEventArgs> PanAndZoomEndedEvent =
        RoutedEvent.Register<PanAndZoomEndedEventArgs>(
            "PanAndZoomEnded", RoutingStrategies.Bubble, typeof(PanAndZoomGestureRecognizer));

    private float _initialDistance;
    private IPointer? _firstContact;
    private Point _firstPoint;
    private IPointer? _secondContact;
    private Point _secondPoint;
    private Point _previousOrigin;

    protected override void PointerCaptureLost(IPointer pointer)
    {
        RemoveContact(pointer);
    }

    protected override void PointerMoved(PointerEventArgs e)
    {
        if (Target is Visual visual)
        {
            if (_firstContact == e.Pointer)
            {
                _firstPoint = e.GetPosition(visual);
            }
            else if (_secondContact == e.Pointer)
            {
                _secondPoint = e.GetPosition(visual);
            }
            else
            {
                return;
            }

            if (_firstContact != null && _secondContact != null)
            {
                var distance = GetDistance(_firstPoint, _secondPoint);

                var scale = distance / _initialDistance;

                var origin = (_firstPoint + _secondPoint) / 2;

                var pinchEventArgs = new PanAndZoomEventArgs(scale, origin, origin - _previousOrigin);
                _previousOrigin = origin;

                Target?.RaiseEvent(pinchEventArgs);
                e.Handled = pinchEventArgs.Handled;
                e.PreventGestureRecognition();
            }
        }
    }

    protected override void PointerPressed(PointerPressedEventArgs e)
    {
        if (Target is Visual visual && (e.Pointer.Type == PointerType.Touch || e.Pointer.Type == PointerType.Pen))
        {
            if (_firstContact == null)
            {
                _firstContact = e.Pointer;
                _firstPoint = e.GetPosition(visual);

                return;
            }
            else if (_secondContact == null && _firstContact != e.Pointer)
            {
                _secondContact = e.Pointer;
                _secondPoint = e.GetPosition(visual);
            }
            else
            {
                return;
            }

            if (_firstContact != null && _secondContact != null)
            {
                _initialDistance = GetDistance(_firstPoint, _secondPoint);

                _previousOrigin = new Point((_firstPoint.X + _secondPoint.X) / 2.0f, (_firstPoint.Y + _secondPoint.Y) / 2.0f);

                Capture(_firstContact);
                Capture(_secondContact);
                e.PreventGestureRecognition();
            }
        }
    }

    protected override void PointerReleased(PointerReleasedEventArgs e)
    {
        if(RemoveContact(e.Pointer))
        {
            e.PreventGestureRecognition();
        }
    }

    private bool RemoveContact(IPointer pointer)
    {
        if (_firstContact == pointer || _secondContact == pointer)
        {
            if (_secondContact == pointer)
            {
                _secondContact = null;
            }

            if (_firstContact == pointer)
            {
                _firstContact = _secondContact;

                _secondContact = null;
            }

            Target?.RaiseEvent(new PanAndZoomEndedEventArgs());
            return true;
        }
        return false;
    }

    private static float GetDistance(Point a, Point b)
    {
        var length = b - a;
        return (float)new Vector(length.X, length.Y).Length;
    }
}